Background. Commonly occurring diseases can have the origin in oxidative processes ongoing in the human body. Vegetables of Brassicaceae family are the essential sources of natural antioxidants, especially phenolic compounds, in the human diet. The research was aimed to estimate the content of phenolic compounds in se- lected vegetables and their quantity changes during hydrothermal processes.Material and methods. The vegetables subjected to analysis were: kale, broccoli, Brussels sprouts, and white and green cauliflower. The fresh and processed (blanched, cooked, frozen, cooked after freezing) vegetables were freeze-dried. The levels of phenolic acids and flavonols by HPLC method were estimated.Results. The presence of derivatives of hydroxycinnamic acid, mainly of caffeic acid, p-coumaric acid, sinapic acid and of flavonols – kaempferol, and in smaller amounts of quercetin was found. The largest amounts of above components were present in kale (total 94.4 mg•100 g-1 of fresh matter), whereas the smallest amounts were found in white and green cauliflower – 3.6 mg•100 g-1 f.m. and 3.03 mg•100 g-1 f.m., respectively. The applied technological processes contributed to lover amounts of all tested compounds depending on the process and the vegetable kind. The biggest loses, up to 70-80%, took place during cooking of raw and previously frozen vegetables.Conclusions. Analysed Brassicaceae were characterized by high contents of the investigated flavonoids. The best source of those compounds was kale whereas the smallest amounts of searched components were presented in cauliflowers. The used hydrothermal processes led to losses of searched compounds.